A solder mask is a coating. It is applied to printed circuits to prevent molten solder from adhering to select portions of the printed circuit. Typically, the solder mask is applied using conventional screen printing techniques. This enables the entire surface of the board to be covered except for connection locations such as holes, pads and contact fingers. After soldering, the solder adheres only to the connection locations because the solder mask covers other locations. Without a solder mask, solder would not only stick to the dielectric substrate but also bridge across that substrate and short circuit conductors. Through use of the solder mask, the ability of molten solder to adhere to the circuit board surface is greatly reduced.
Typically, a solder mask is applied only to the solder side of the circuit board. It can be applied to both sides.
Typically, there are four kinds of solder masks: (1) two part epoxy, (2) one part epoxy, (3) ultraviolet curable and (4) dry film solder masks. These are screen printed onto a circuit board except for the latter mask. That dry film mask is laminated onto the circuit board surface exposed with a photo tool and UV light, and developed like photoresist.
Two part epoxy solder masks must be weighed and mixed thoroughly before they are screen printed. If this is not done, the mask may have streaky discoloration and poor adhesion during wave soldering. This mask is cured to hardness, heat resistance and chemical resistance by baking in a forced convection oven.
The one part mask does not require mixing. Due to its high viscosity, it is more difficult to print. The material also cures by oven baking, but the baking time is about double compared to the two part epoxy solder mask. With both epoxy masks, they must be cured sufficiently to avoid white spots or a white film appearing on their surface after wave soldering.
UV curable masks offer shorter curing times and low volatile content. The masks are screen printed. Then, they are run through a conveyorized ultraviolet light source for curing. Some baking facilitates completion of the cure. The low volatiles content avoids having to remove solvent during the curing cycle. Using this mask helps avoid contamination. However, UV curable masks are expensive, require excellent exposure control and require excellent cleanliness and dryness of the circuit board before the mask is applied. Otherwise, disadvantages occur, specifically, blistering of the mask during wave soldering and poor adhesion.
Dry film solder masks involve acrylic photopolymer technology. Their main advantages are utilization in tightly spaced conductors, finer registration of the mask to the board, absence of bleeding, and enhanced mechanical and electrical protection for the printed circuit. This kind of mask also has disadvantages including criticality of surface cleanliness and dryness to promote adhesion, expense, complicated use, substantial expense for laminating equipment, and critical exposure control.
Solder masks are more difficult to print than are plating resist. The material has a high viscosity and is printed over a raised surface, that is, the plated circuitry. Solder masks require greater pressure on the stencil than is required during a typical screen printing operation. The printing strokes should be slow enough to flood between the circuit traces. A coarse screening mesh should be used during printing. Exposure of the stencil is about 25% longer than is required for printing plating resist. This extra exposure time makes the stencil a little thicker and stronger.
Sometimes bleeding occurs and the solder mask flows onto the pads. This may require blotting the stencil periodically. Moreover, inspection of each panel is required before the mask is applied. This is necessary because the solder mask can not be stripped without damaging the surface of the printed circuit. See, R. Clark, Handbook Of Printed Circuit Manufacturing, pp. 241-244 (1985), hereby incorporated by reference.
The present invention overcomes disadvantages of the prior art while providing improved adhesion and flexibility to the printed circuit and impairs growth in the barrier layer. The flexibility and adhesion characteristics permit the mask to be applied to a transfer medium above or combined with other layers for subsequent lamination to a substrate having a complex surface configuration. Applicants' solder mask composition is particularly useful with thermoplastic substrates which cannot utilize conventional epoxy solder masks.